Hydrologic Units of the United States

April 28, 2026

HUC diagram showing the hierarchical, nested way in which HUCs are named — This image illustrates this nesting of hydrologic unit codes (HUCs), which result in successively smaller drainage units and add digits to the HUC to create a unique “address.”

The U.S. Geological Survey uses a depiction and classification scheme for hydrologic units known as hydrologic unit codes (HUCs). HUCs generally represent catchments, and river basins are represented by a unique series of numbers with successively smaller hydrologic units nested inside of larger ones. Digits are added as hydrologic units become smaller, such that a 2-digit HUC encompasses multiple 4-digit HUCs, and a 4-digit HUC encompasses multiple 6-digit HUCs, etc. Visit the Hydrologic Units of the United States website to learn more and explore HUCs through interactive maps.

Although in practice, HUCs represent river basins, they are formally administrative units, and not always natural watersheds. As such, HUCs can depict human-created drainages and diversions or arbitrary breaks in connected flow systems.